In the field of optical communication, a communication system using a combination of a coherent detection scheme and signal processing for dramatically improving frequency utilization efficiency has attracted attention. This type of communication system can improve reception sensitivity, compared to a system built with direct detection. In addition, such a communication system is known to receive a transmitted signal as a digital signal, so that the communication system can perform signal position detection, frequency offset compensation, clock offset compensation, chromatic dispersion compensation, PMD (polarization-mode dispersion) compensation, and the like through signal processing of the received digital signal, and to have a high tolerance to signal quality degradation due to non-linear effects. Accordingly, introduction of such a communication system as a next-generation optical communication technique has been examined.
For example, in a wireless communication 802.11a standard, synchronization can be established by estimating a frequency offset or a clock offset using a configuration in which a short preamble or a long preamble is inserted into a transmission signal and by compensating for the offset based on the estimated result (see Non-Patent Document 1).
Further, in the field of conventional optical fiber communication, signal demodulation is performed by an analog differential detection process. Further, in a conventional optical transmission using a direct detection scheme, degradation of the signal quality is reduced using an optical compensator, such as a dispersion compensation fiber, and an electrical analog equalizer with respect to signal quality degradation factors for a transmission line such as chromatic dispersion and polarization-mode dispersion.
In recent years, a coherent transmission scheme using digital signal processing has been actively studied. This coherent transmission scheme can increase reception sensitivity, compared to the direct detection scheme. Further, the coherent transmission scheme is capable of accurately equalizing waveform distortion due to chromatic dispersion, polarization-mode dispersion, or the like by performing digital signal processing in a receiver. This dramatically increases a transmission distance of 100 Gb/s/ch optical signal particularly sensitive to the waveform distortion.
Digital coherent schemes exemplified in Non-Patent Documents 2 and 3 employ a method in which quasi-static chromatic dispersion is compensated for by a digital filter having a fixed number of taps (e.g., the number of taps is 2048 for the dispersion of 20000 ps/nm and for a signal at 28 G baud), and variable polarization-mode dispersion is compensated for by an adaptive filter with a small number of taps (e.g., about 10 to 12 taps for polarization-mode dispersion of 50 ps) using a blind algorithm.
In addition, polarization-division multiplexing transmission accompanying a high transmission rate has attracted attention, as exemplified in Non-Patent Document 4.